WO2022138686A1

WO2022138686A1 - Cellulose nanofiber composition and production method therefor

Info

Publication number: WO2022138686A1
Application number: PCT/JP2021/047465
Authority: WO
Inventors: 浩介黒田; 裕人添田; 圭樹伊藤; 洋介後居
Original assignee: 国立大学法人金沢大学; 第一工業製薬株式会社
Priority date: 2020-12-25
Filing date: 2021-12-22
Publication date: 2022-06-30
Also published as: JP2022102007A

Abstract

A purpose of the present invention is to provide a cellulose nanofiber composition which, in a dried state, has excellent redispersibility. The cellulose nanofiber composition comprises modified cellulose nanofibers having an ionic functional group and a betaine compound having a molecular weight of 600 or less. The betaine compound is contained in an amount of preferably 5-1,000 parts by mass per 100 parts by mass of the modified cellulose nanofibers.

Description

Cellulose nanofiber composition and its manufacturing method

The present invention relates to a cellulose nanofiber composition and a method for producing the same.

Cellulose nanofiber (CNF) is a fibrous substance obtained by chemically and mechanically processing cellulose raw materials such as pulp obtained from wood and defibrating them to nano size. While having the same strength as steel, this cellulose nanofiber is characterized by being lightweight (one-fifth the weight of steel), and has a low thermal expansion rate and a size finer than the wavelength of visible light. Taking advantage of its high recyclability, safe and secure natural products, it can be used for various purposes such as structural materials such as exteriors of cars and home appliances, optical materials, separation materials such as filters, and thickeners in cosmetics. Is expected to be applied.

Cellulose nanofibers are usually produced by refining pulp or the like dispersed in water. The obtained cellulose nanofibers are in a water-dispersed state, and such a dispersion has a problem that the transportation cost is high.

On the other hand, when the cellulose nanofibers are dried, they are strongly aggregated due to hydrogen bonds between the cellulose nanofibers, so that when the dried product is redispersed in water, the dispersibility does not sufficiently return to the state before drying. There is an inconvenience. In addition, cellulose nanofibers have the disadvantage that their performance deteriorates when they aggregate.

Therefore, in order to improve the dispersibility of the dried product, (1) the cellulose nanofibers are modified (Patent Documents 1 and 2), and (2) an aggregation inhibitor is added when the dispersion liquid of the cellulose nanofibers is dried. Measures such as (Patent Document 3) have been attempted. For example, Patent Document 3 discloses a cellulose nanofiber-containing dry body containing cellulose nanofibers, a redispersing agent such as glycerin, and a redispersion accelerator which is a salt. However, all of the conventional techniques are insufficient in terms of redispersibility of cellulose nanofibers and applicability to various uses, and there is still room for improvement.

Japanese Patent No. 5381338 Japanese Patent No. 5500842 Japanese Unexamined Patent Publication No. 2018-9134

Therefore, in view of the above-mentioned conventional situation, it is an object of the present invention to provide a cellulose nanofiber composition having excellent dispersibility when a dried product is dispersed again, and a method for producing the same.

The present inventors added betaine compounds having a molecular weight of 600 or less, such as trimethylglycine, to modified cellulose nanofibers having an ionic functional group, and the nanofibers were redispersed when the dried cellulose nanofibers were redispersed. The invention was completed by finding that the aggregation of cellulose was suppressed. That is, the gist of the present invention is as follows.

(1) A cellulose nanofiber composition containing a modified cellulose nanofiber having an ionic functional group and a betaine compound having a molecular weight of 600 or less.
(2) The cellulose nanofiber composition according to (1) above, wherein the content of the betaine compound is 5 to 1000 parts by mass with respect to 100 parts by mass of the modified cellulose nanofibers.
(3) The cellulose nanofiber composition according to (1) or (2) above, which is a dried product.
(4) The cellulose nanofiber composition according to (1) or (2) above, which comprises a dispersion medium and in which the modified cellulose nanofibers are dispersed in the dispersion medium.
(5) A method for producing a dry cellulose nanofiber composition, which comprises a step of drying a dispersion of modified cellulose nanofibers having an ionic functional group and containing a betaine compound having a molecular weight of 600 or less.
(6) The method for producing a cellulose nanofiber composition according to (5) above, wherein the drying step is a heat drying step or a freeze drying step.

According to the cellulose nanofiber composition of the present invention, the hydrogen bond between the cellulose nanofibers at the time of drying is inhibited by the betaine compound having a molecular weight of 600 or less, and the redispersibility of the dried product can be enhanced. Further, since the betaine compound is an additive that is safe for living organisms, the cellulose nanofiber composition can be used for various purposes.

It is a figure which shows the AFM image of the redispersion of Example 8. It is a figure which shows the AFM image of the redispersion of Example 9. It is a figure which shows the AFM image of the redispersion of the comparative example 4. It is a figure which shows the AFM image of the redispersion of Example 10. It is a figure which shows the AFM image of the redispersion of the comparative example 5.

Hereinafter, the present invention will be described in detail based on the embodiments.
The cellulose nanofiber composition according to the present embodiment contains modified cellulose nanofibers having an ionic functional group and a betaine compound having a molecular weight of 600 or less.

The modified cellulose nanofiber is obtained by defibrating a cellulose raw material to a nano size by, for example, a method described later. The modified cellulose nanofibers preferably have a number average fiber diameter of 2 nm or more and 500 nm or less, a fiber aspect ratio of 50 or more and 1000 or less, and have a cellulose I-type crystal structure, but are not limited thereto.

The number average fiber diameter is more preferably 2 nm or more and 150 nm or less, and further preferably 2 nm or more and 100 nm or less. Particularly preferably, it is 2 nm or more and 50 nm or less. The maximum fiber diameter of the modified cellulose nanofibers is preferably 1000 nm or less, and particularly preferably 500 nm or less when transparency after drying is required.

The number average fiber diameter and the maximum fiber diameter of the modified cellulose nanofibers can be measured, for example, as follows. That is, an aqueous dispersion of fine cellulose having a solid content of 0.05 to 0.1% by weight was prepared, and the dispersion was cast on a carbon film-coated grid having been hydrophilized, and a transmission electron microscope was used. (TEM) observation sample. When a fiber having a large fiber diameter is included, a scanning electron microscope (SEM) image of the surface cast on the glass may be observed. Then, observation is performed using an electron microscope image at a magnification of 5000 times, 10000 times, or 50,000 times depending on the size of the constituent fibers. At that time, an axis having an arbitrary vertical and horizontal image width is assumed in the obtained image, and the sample and observation conditions (magnification, etc.) are adjusted so that 20 or more fibers intersect the axis. Then, after obtaining an observation image satisfying this condition, two random axes are drawn vertically and horizontally for each image, and the fiber diameters of the fibers intersecting the axes are visually read. In this way, images of at least three non-overlapping surface portions are taken with an electron microscope and the value of the fiber diameter of the fibers intersecting each of the two axes is read (hence, at least 20 fibers × 2 × 3 = 120). Information on the fiber diameter of the book can be obtained). From the fiber diameter data obtained in this way, the maximum fiber diameter and the number average fiber diameter are calculated.

The average aspect ratio of the modified cellulose nanofibers is more preferably 100 or more and 1000 or less, and particularly preferably 200 or more and 1000 or less. If the average aspect ratio is less than 50, the strength of the fiber itself may decrease.

The average aspect ratio of the modified cellulose nanofibers is determined by the following formula from the number average fiber diameter measured as described above and the number average fiber lengths of the 20 modified cellulose nanofibers similarly measured from the TEM image. It can be calculated according to (1).
Average aspect ratio = number average fiber length (nm) / number average fiber diameter (nm) (1)

The modified cellulose nanofiber is a fiber obtained by refining a naturally-derived cellulose raw material having an I-type crystal structure. That is, in the process of biosynthesis of natural cellulose, nanofibers called microfibrils are first formed almost without exception, and these are multi-bundle to form a high-order solid structure.

The fact that the cellulose constituting the modified cellulose nanofiber has an I-type crystal structure is that, for example, in the diffraction profile obtained by wide-angle X-ray diffraction image measurement, 2θ = 14 to 17 ° and 2θ = 22 to 23 °. It can be identified by having typical peaks at the two positions of.

The modified cellulose nanofiber can be obtained from, for example, natural cellulose as a raw material.

The natural cellulose is not particularly limited as long as it is a cellulose derived from plants, animals or microorganisms, and is kraft pulp or dissolving pulp derived from coniferous or broadleaf trees, cotton linter, lignocellulose with low cellulose purity, wood flour, vegetation cellulose, bacterial cellulose. Etc. are applicable.

In addition, bacterial cellulose produced by bacteria can be used as natural cellulose. Examples of the above-mentioned bacteria include the genus Acetobacter, and more specifically, Acetobacter aceti, Acetobacter subsp., Acetobacter xylinum and the like. Can be mentioned. By culturing these bacteria, cellulose is produced from the bacteria. Since the obtained product contains a bacterium and cellulose (bacterial cellulose) produced from this bacterium and linked to the bacterium, this product is taken out from the medium and washed with water or treated with an alkali. By removing the bacteria, a water-containing bacterial cellulose containing no bacteria can be obtained.

In the present embodiment, an ionic functional group is introduced into the cellulose nanofiber and modified. The modification with an ionic functional group may be either anion modification or cationic modification. Examples of the anion modification include carboxylation, carboxymethylation, phosphoric acid esterification, subphosphate esterification, and sulfate esterification, and examples of the cationic modification include amination. Since the betaine compound described later has both positive and negative charges, it acts similarly on modified cellulose nanofibers having an ionic functional group regardless of whether it is an anion or a cation. The redispersibility of the obtained dried product can be improved.

In particular, it is preferable that the ionic functional group is an anionic functional group. The anion-modified cellulose is not particularly limited, and specific examples thereof include oxidized cellulose, carboxymethyl cellulose, multivalent carboxymethyl cellulose, and long-chain carboxycellulose. Among these, oxidized cellulose is preferable because it has excellent selectivity of hydroxyl groups on the fiber surface and the reaction conditions are mild. Carboxymethyl cellulose is also preferable from the viewpoint of versatility and safety.

The oxidized cellulose is made from natural cellulose, an N-oxyl compound is used as an oxidation catalyst in water, and an oxidizing agent is allowed to act on the natural cellulose to oxidize the natural cellulose to obtain a reactant fiber. It can be obtained by a production method including a purification step of obtaining a reaction product fiber impregnated with water and a dispersion step of dispersing the reaction product fiber impregnated with water in a solvent.

In the oxidized cellulose, it is preferable that the hydroxyl group at the C6 position of each glucose unit in the cellulose molecule is selectively oxidatively modified to become one of an aldehyde group, a ketone group, and a carboxyl group. The content of the carboxyl group (amount of carboxyl group) is preferably in the range of 1.2 to 2.5 mmol / g, more preferably in the range of 1.5 to 2.0 mmol / g from the viewpoint of dispersibility in water.

For the measurement of the amount of carboxyl groups of the oxidized cellulose, for example, 60 ml of 0.5 to 1% by weight slurry was prepared from a cellulose sample whose dry weight was precisely weighed, and the pH was adjusted to about 2.5 with a 0.1 M aqueous hydrochloric acid solution. After that, a 0.05 M aqueous solution of sodium hydroxide is added dropwise to measure the electrical conductivity. The measurement is continued until the pH reaches about 11. From the amount of sodium hydroxide (V) consumed in the neutralization step of a weak acid with a gradual change in electrical conductivity, the amount of carboxyl group can be determined according to the following formula (2).
Carboxyl group amount (mmol / g) = V (ml) x [0.05 / cellulose weight] (2)

The amount of the carboxyl group can be adjusted by controlling the addition amount of the copolymer and the reaction time used in the oxidation step of the cellulose nanofibers, as described later.

It is preferable that the oxidized cellulose is reduced with a reducing agent after oxidative denaturation. As a result, a part or all of the aldehyde group and the ketone group are reduced and returned to the hydroxyl group. The carboxyl group is not reduced. Then, by the reduction, the total content of the aldehyde group and the ketone group of the cellulose nanofibers as measured by the semicarbazide method is preferably 0.3 mmol / g or less, and particularly preferably 0 to 0.1 mmol / g. The range of, most preferably substantially 0 mmol / g. This makes it possible to suppress a decrease in the molecular weight of the cellulose nanofibers.

The oxidized cellulose is oxidized with a copolymer in the presence of an N-oxyl compound such as 2,2,6,6-tetramethylpiperidin (TEMPO), and is an aldehyde group generated by the oxidation reaction. And it is preferable that the ketone group is reduced by a reducing agent because the cellulose nanofibers can be easily obtained. Further, it is preferable that the reduction with the reducing agent is with sodium borohydride (NaBH ₄ ).

The measurement of the total content of the aldehyde group and the ketone group by the semicarbazide method is performed, for example, as follows. That is, exactly 50 ml of a 3 g / l aqueous solution of semicarbazide hydrochloride adjusted to pH = 5 with a phosphate buffer solution is added to the dried sample, the sample is sealed, and the mixture is shaken for 2 days. Subsequently, 10 ml of this solution is accurately collected in a 100 ml beaker, 25 ml of 5N sulfuric acid and 5 ml of a 0.05 N potassium iodate aqueous solution are added, and the mixture is stirred for 10 minutes. Then, 10 ml of a 5% potassium iodide aqueous solution is added, and the titration is immediately performed with a 0.1N sodium thiosulfate solution using an automatic titration device. The amount of carbonyl group (total content of aldehyde group and ketone group) can be determined. Semicarbazide reacts with an aldehyde group or a ketone group to form a Schiff base (imine), but does not react with a carboxyl group. Therefore, it is considered that only the aldehyde group and the ketone group can be quantified by the above measurement.
Carbonyl group amount (mmol / g) = (DB) × f × [0.125 / w] (3)
D: Sample titration (ml)
B: Titration of blank test (ml)
f: Factor of 0.1N sodium thiosulfate solution (-)
w: Sample amount (g)

In the oxidized cellulose, the hydroxyl group at the C6 position of each glucose unit in the cellulose molecule on the fiber surface is selectively oxidatively modified to become either an aldehyde group, a ketone group or a carboxyl group. Whether or not only the hydroxyl group at the C6 position of the glucose unit on the surface of the cellulose nanofibers is selectively oxidized can be confirmed by, for example, a ¹³ C-NMR chart. That is, the peak of 62 ppm corresponding to the C6 position of the primary hydroxyl group of the glucose unit, which can be confirmed on the ¹³ C-NMR chart of cellulose before oxidation, disappears after the oxidation reaction, and instead the peak derived from the carboxyl group or the like (178 ppm). The peak of is derived from the carboxyl group). In this way, it can be confirmed that only the C6-position hydroxyl group of the glucose unit is oxidized to the carboxyl group or the like.

Further, the detection of the aldehyde group in the oxidized cellulose can also be performed by, for example, a Fehling's reagent. That is, for example, when a Fehling's reagent (a mixed solution of sodium potassium tartrate and sodium hydroxide and an aqueous solution of copper sulfate pentahydrate) is added to a dried sample and then heated at 80 ° C. for 1 hour, the supernatant becomes clear. If the blue or cellulose nanofiber part is dark blue, it can be judged that the aldehyde group was not detected, and if the supernatant is yellow and the cellulose fiber part is red, it can be judged that the aldehyde group was detected. can.

The modified cellulose nanofibers obtained by defibrating the oxidized cellulose are preferably produced by (1) oxidation reaction step, (2) reduction step, (3) purification step, (4) dispersion step (micronization treatment step) and the like. Specifically, it is preferable to manufacture by each of the following steps.

(1) Oxidation reaction step After dispersing natural cellulose and an N-oxyl compound in water (dispersion medium), a copolymer is added to start the reaction. During the reaction, a 0.5 M aqueous sodium hydroxide solution is added dropwise to keep the pH at 10 to 11, and the reaction is considered to be completed when no change in pH is observed. Here, the copolymer is not a substance that directly oxidizes the cellulose hydroxyl group, but a substance that oxidizes an N-oxyl compound used as an oxidation catalyst.

The natural cellulose means purified cellulose isolated from the biosynthetic system of cellulose such as plants, animals, and bacterial gels. More specifically, softwood pulp, broadleaf pulp, cotton linter, cotton lint and other cotton pulp, straw pulp, bagus pulp and other non-wood pulp, bacterial cellulose (BC), cellulose isolated from squirrel, seaweed. Examples thereof include cellulose isolated from. These may be used alone or in combination of two or more. Among these, softwood-based pulp, hardwood-based pulp, cotton-based pulp such as cotton linter and cotton lint, and non-wood-based pulp such as straw pulp and bagas pulp are preferably used. It is preferable that the natural cellulose is subjected to a treatment for increasing the surface area such as beating because the reaction efficiency can be increased and the productivity can be increased. In addition, if the natural cellulose that has been isolated, purified, and then stored without drying (never dry) is used, the aggregates of microfibrils are in a state of being easily swollen, so that the reaction efficiency is improved and the particles are fine. It is preferable because the number average fiber diameter after the conversion treatment can be reduced.

The dispersion medium of natural cellulose in the reaction is water, and the concentration of natural cellulose in the reaction aqueous solution is arbitrary as long as the reagent (natural cellulose) can be sufficiently diffused. Normally, it is about 5% or less based on the weight of the reaction aqueous solution, but the reaction concentration can be increased by using a device having a strong mechanical stirring force.

Further, examples of the N-oxyl compound include compounds having a nitroxy radical, which is generally used as an oxidation catalyst. The N-oxyl compound is preferably a water-soluble compound, preferably a piperidine nitroxyoxy radical, particularly 2,2,6,6-tetramethylpiperidinooxy radical (TEMPO) or 4-acetamide-TEMPO. preferable. The amount of the catalyst is sufficient for the addition of the N-oxyl compound, and the N-oxyl compound is preferably added to the reaction aqueous solution in the range of 0.1 to 4 mmol / l, more preferably 0.2 to 2 mmol / l.

Examples of the copolymer include hypohalogenic acid or a salt thereof, subhalogenic acid or a salt thereof, perhalogenic acid or a salt thereof, hydrogen peroxide, a perorganic acid and the like. These may be used alone or in combination of two or more. In particular, alkali metal hypohalites such as sodium hypochlorite and sodium hypobromous acid are preferable. When the sodium hypochlorite is used, it is preferable to proceed the reaction in the presence of an alkali bromide metal such as sodium bromide from the viewpoint of the reaction rate. The amount of the alkali metal bromide added is about 1 to 40 times the molar amount, preferably about 10 to 20 times the molar amount of the N-oxyl compound.

The pH of the reaction aqueous solution is preferably maintained in the range of about 8 to 11. The temperature of the aqueous solution can be arbitrarily set within the range of about 4 to 40 ° C., but the reaction can be carried out at room temperature (25 ° C.), and no particular temperature control is required. In order to obtain a desired amount of carboxyl groups and the like, the degree of oxidation is controlled by the amount of the copolymer added and the reaction time. Reaction times are usually about 5 to 120 minutes and are usually completed within 240 minutes at the longest.

(2) Reduction Step It is preferable that the oxidized cellulose is further subjected to a reduction reaction after the oxidation reaction. Specifically, the fine cellulose oxide after the oxidation reaction is dispersed in purified water, the pH of the aqueous dispersion is adjusted to about 10, and the reduction reaction is carried out with various reducing agents. As the reducing agent used in the present invention, general ones can be used, but LiBH ₄ , NaBH ₃ CN, NaBH ₄ , and the like are preferable. In particular, NaBH ₄ is preferably used in terms of cost and availability.

The amount of the reducing agent is preferably in the range of 0.1 to 4% by weight, particularly preferably in the range of 1 to 3% by weight, based on the fine cellulose oxide. The reaction is usually carried out at room temperature or a temperature slightly higher than room temperature for 10 minutes to 10 hours, preferably 30 minutes to 2 hours.

After the above reaction is completed, the pH of the reaction mixture is adjusted to about 2 with various acids, and solid-liquid separation is performed with a centrifuge while sprinkling purified water to obtain cake-shaped fine oxidized cellulose. The solid-liquid separation is carried out until the electric conductivity of the filtrate becomes 5 mS / m or less.

(3) Purification step Next, purification is performed for the purpose of removing unreacted copolymers (hypochlorous acid, etc.) and various by-products. Since the reactant fibers are not usually dispersed in nanofiber units at this stage, they can be obtained with high-purity (99% by weight or more) reactant fibers by repeating the usual purification method, that is, washing with water and filtration. Obtain a dispersion of water.

The purification method in the purification step may be any device as long as it can achieve the above-mentioned object, such as a method using centrifugal dehydration (for example, a continuous decander). The aqueous dispersion of the reactant fibers thus obtained has a solid content (cellulose) concentration in the range of about 10% by weight to 50% by weight in a squeezed state. Considering the subsequent dispersion step, if the solid content concentration is higher than 50% by weight, extremely high energy is required for dispersion, which is not preferable.

(4) Dispersion process (miniaturization process)
The water-impregnated reactant fiber (aqueous dispersion) obtained in the purification step is dispersed in a dispersion medium and subjected to a dispersion treatment. The viscosity increases with the treatment, and a dispersion of modified cellulose nanofibers having a finely divided ionic functional group can be obtained. Then, if necessary, the modified cellulose nanofibers may be dried, and as a method for drying the dispersion of the modified cellulose nanofibers, for example, when the dispersion medium is water, a spray drying method or a freeze drying method may be used. A vacuum drying method or the like is used, and when the dispersion medium is a mixed solution of water and an organic solvent, a drying method using a drum dryer, a spray drying method using a spray dryer, or the like is used. The dispersion of the modified cellulose nanofibers having an ionic functional group may be used in the state of the dispersion without drying.

Dispersors used in the dispersion step include homomixers, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersers, beaters, disc-type refiners, conical-type refiners, double-disc-type refiners, grinders, etc. under high-speed rotation. It is preferable to use a powerful and beating ability device in that more efficient and advanced downsizing is possible and the dispersion can be economically advantageous. Examples of the disperser include a screw type mixer, a paddle mixer, a disper type mixer, a turbine type mixer, a disper, a propeller mixer, a kneader, a blender, a homogenizer, an ultrasonic homogenizer, a colloid mill, a pebble mill, and a bead mill crusher. You can use it. Further, two or more types of dispersers may be used in combination.

Further, carboxymethyl cellulose, which is one of the anion-modified modified cellulose nanofibers, can be produced by the following method using the cellulose raw material. That is, using cellulose as a raw material, the solvent is a lower alcohol 3 to 20 times by mass, specifically, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol, etc. alone. , Or a mixed medium of two or more mixtures and water. The mixing ratio of the lower alcohol is 60 to 95% by mass. As the mercerizing agent, 0.5 to 20 times mol of alkali metal hydroxide per glucose residue of cellulose, specifically sodium hydroxide or potassium hydroxide is used. Cellulose, solvent and mercerizing agent are mixed to perform mercerization treatment. The reaction temperature at this time is 0 to 70 ° C., preferably 10 to 60 ° C., and the reaction time is 15 minutes to 8 hours, preferably 30 minutes to 7 hours. Then, a carboxymethylating agent is added in an amount of 0.05 to 10 times per glucose residue to carry out an etherification reaction. The reaction temperature at this time is 30 to 90 ° C., preferably 40 to 80 ° C., and the reaction time is 30 minutes to 10 hours, preferably 1 hour to 4 hours.

Modified cellulose nanofibers can be obtained by defibrating the carboxymethyl cellulose with a high-pressure homogenizer or the like. A high-pressure homogenizer is a device that pressurizes a fluid with a pump and ejects it from a very delicate gap provided in a flow path. Emulsification, dispersion, defibration, crushing, and ultrafineness can be performed by total energy such as collision between particles and shearing force due to pressure difference.

The treatment conditions by the homogenizer of the present invention are not particularly limited, but the pressure conditions are 30 MPa or more, preferably 100 MPa or more, and more preferably 140 MPa or more. It is also possible to pre-treat carboxymethyl cellulose using a known mixing, stirring, emulsifying and dispersing device such as a high-speed shear mixer, if necessary, prior to the defibration / dispersion treatment with a high-pressure homogenizer. be.

The degree of carboxymethyl substitution per glucose unit of carboxymethyl cellulose is preferably 0.02 or more and 0.50 or less. By introducing a carboxymethyl substituent into cellulose as an ionic functional group, the celluloses electrically repel each other. Therefore, cellulose having a carboxymethyl substituent introduced can be easily nano-deflated. The ratio of carboxymethyl substituents per glucose unit is preferably in the range of 0.02 to 0.50.

In the present embodiment, the betaine compound contained in the cellulose nanofiber composition together with the modified cellulose nanofiber having an ionic functional group includes trimethylglycine (betaine in a narrow sense) and positive and negative charges in the same molecule. It refers to a compound that has no charge as a whole molecule because hydrogen that can be dissociated is not bonded to the atoms that are not adjacent to each other and have a positive charge. The molecular weight of the betaine compound is 600 or less. The cation moiety in the betaine compound can have a structure such as quaternary ammonium, sulfonium, or phosphonium. Such betaine compounds electrically bond to the ionic functional groups of the modified cellulose nanofibers and uniformly coat the fiber surface of the modified cellulose nanofibers. Therefore, it is considered that hydrogen bonds between modified cellulose nanofibers during drying are inhibited and aggregation is suppressed, but the mechanism of action for improving redispersibility is not bound by this theory.

Examples of betaine compounds include trimethylglycine, L-carnitine, D-carnitine, proline betaine, lauryldimethylaminoacetic acid betaine, coconut oil fatty acid amide propyl betaine, etc., either alone or in combination of two. The above can be used in combination. In particular, trimethylglycine is preferably used.

In the present embodiment, the blending ratio of the modified cellulose nanofibers and the betaine compound in the cellulose nanofiber composition is appropriately considered in consideration of the properties of the modified cellulose nanofibers (fiber diameter, type of modification, etc.) and the use of the composition. Can be set. Specifically, the content of the betaine compound with respect to 100 parts by mass of the modified cellulose nanofibers is preferably 5 to 1000 parts by mass, and the content of the betaine compound with respect to 100 parts by mass of the modified cellulose nanofibers is 10 to 500 parts by mass. It is more preferably 20 to 200 parts by mass.

The cellulose nanofiber composition can be a dispersion in which modified cellulose nanofibers and a betaine compound are dispersed in a medium. The medium (that is, a dispersion medium) is preferably an aqueous medium, specifically, an aqueous solution of a lower alcohol such as water, methyl alcohol or ethyl alcohol, an aqueous solution of glycol such as ethylene glycol or propylene glycol, or saturation of D-sorbitol. Examples thereof include a chain hydrocarbon-based polyhydric alcohol aqueous solution and other organic compound aqueous solutions, and an inorganic salt aqueous solution such as calcium chloride and sodium chloride, but the present invention is not particularly limited thereto. Among these, water, a lower alcohol aqueous solution such as methyl alcohol and ethyl alcohol, and a glycol aqueous solution such as ethylene glycol and propylene glycol are preferable, and water is particularly preferable. When the medium in which the modified cellulose nanofibers are dispersed is a mixture of water and an aqueous medium other than water, the ratio of water to the entire medium is not particularly limited, but is, for example, 10% by mass or more. Is more preferable, 40% by mass or more is more preferable, and 60% by mass or more is further preferable.

Further, the cellulose nanofiber composition can be a dried product. When dried, the fiber surface of the modified cellulose nanofibers is covered with the betaine compound, and aggregation is suppressed, so that the redispersibility is excellent, and the dried state has advantages such as low transportation cost.

The cellulose nanofiber composition which is a dried product is produced by subjecting a modified cellulose nanofiber having an ionic functional group to a medium such as water in which a predetermined amount of betaine compound is added and dried. Can be done. The molecular weight of the betaine compound is 600 or less. The medium such as water may be completely removed by drying, but the medium may be partially left depending on the use of the composition and the like. Further, as the means for drying, either heat drying or freeze drying can be applied. Conditions such as the drying temperature and time during heat drying can be appropriately set. As an example, when 10 ml of an aqueous dispersion containing 2% by mass of modified cellulose nanofibers is dried, it can be carried out at a temperature of 80 to 120 ° C. for 10 to 60 minutes.

Further, the cellulose nanofiber composition which is a dried product can be suitably redispersed in a dispersion medium as described above, for example. Thereby, the cellulose nanofiber composition containing the dispersion of the modified cellulose nanofiber can be obtained again.

The dispersion of the modified cellulose nanofibers may be liquid or solid, specifically, for example, gel. Further, the dispersion may be in the form of a slurry, for example.

Other additives can be added to the cellulose nanofiber composition according to the present embodiment in consideration of the use of the composition and the like, as long as the effects of the present invention are not impaired. Examples of additives include dispersants, preservatives, defoamers, thickeners, emulsifiers, pH regulators, antioxidants, heat stabilizers, light stabilizers, UV absorbers, pigments, colorants, Flame retardants, plasticizers, fragrances and the like can be mentioned. The content of these other additives is preferably less than 10,000% by mass with respect to the total amount of the modified cellulose nanofibers and the betaine compound.

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Preparation of anion-modified cellulose nanofibers]
To 2.0 g of coniferous kraft pulp, 150 mL of water, 0.25 g of sodium bromide, 0.025 g of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) were added, and after sufficient stirring, 13% hypochlorite was added. An aqueous sodium chlorite solution was added to 1.0 g of the pulp so that the amount of sodium hypochlorite was 6 mmol / g, and the reaction was started. Further, the reaction was carried out for 120 minutes while dropping a 0.5 N aqueous sodium hydroxide solution so as to maintain the pH during the reaction at 10 to 11. After the reaction, 0.1N hydrochloric acid was added to adjust the pH to 2.0, solid-liquid separation was performed by suction filtration, and then pure water was added to the solid content to prepare a slurry having a solid content concentration of 2.0%. Then, the pH was adjusted to 10 with a 10% aqueous sodium hydroxide solution, 0.2 mmol / g of sodium borohydride was added to the cellulose fibers, and the mixture was reacted for 2 hours for reduction treatment. After the reaction, 0.1N hydrochloric acid was added to neutralize the mixture, and filtration and washing with water were repeated for purification. Pure water was added to the obtained purified product to prepare a slurry having a solid content concentration of 2.0%, and then the pH was adjusted to 7 with a 10% aqueous sodium hydroxide solution. Then, as a miniaturization treatment step, a treatment with a microfluitizer (150 MPa, 2 passes) was carried out to obtain anion-modified cellulose nanofibers in which a carboxyl group was introduced at a high density to the cellulose fibers.

The above-mentioned anion-modified cellulose nanofiber has a number average fiber diameter of 3.1 nm, an average aspect ratio of 300, and a carboxyl group is introduced by selectively oxidizing the hydroxyl group at the C6 position. It has a base weight of 2.0 mmol / g and has an I-type crystal structure. Hereinafter, the above-mentioned anion-modified cellulose nanofibers are simply referred to as “cellulose nanofibers”.

(Example 1)
Pure water was added to the above-mentioned aqueous dispersion of cellulose nanofibers and diluted to a solid content concentration of 0.4% by mass. To 100 parts by mass of cellulose nanofibers, 10 parts by mass of trimethylglycine (molecular weight 117.2, manufactured by Wako Pure Chemical Industries, Ltd.) was added to this diluted solution, and the mixture was stirred and dissolved. After allowing this to stand for 1 day, the viscosity was measured using a BM type viscometer (0.6 rpm, 25 ° C., 3 minutes). The obtained value was defined as "viscosity before drying".

The above dispersion was placed in a constant temperature bath at 105 ° C. and allowed to stand until the mass became constant to dry. The obtained dried product was allowed to stand at room temperature for 1 day, and then pure water was added so that the solid content concentration of the cellulose nanofibers became 0.4% by mass. This was stirred at 2,000 rpm for 10 minutes using Homo Disper 2.5 type (manufactured by Primix Corporation) and redispersed. After allowing the redispersion solution to stand for 1 day, the viscosity was measured using a BM type viscometer (0.6 rpm, 25 ° C., 3 minutes). The obtained value was defined as "viscosity after redispersion". Based on the two measured viscosity values, the "redispersability (%)" was defined by the following formula, and the redispersibility of the cellulose nanofibers was evaluated. The results are shown in Table 1.

(Examples 2 to 5)
A dispersion was prepared in the same manner as in Example 1 except that the content of trimethylglycine with respect to the cellulose nanofibers was changed as shown in Table 1, and the redispersibility was evaluated. The results are summarized in Table 1.

(Examples 6 and 7)
A dispersion was prepared in the same manner as in Example 4 except that lauryldimethylaminoacetic acid betaine (molecular weight 271.4) or coconut oil fatty acid amide propyl betaine (molecular weight 342.5) was used instead of trimethylglycine. The redispersibility was evaluated. The results are summarized in Table 1.

(Comparative Example 1)
A dispersion was prepared in the same manner as in Example 1 except that trimethylglycine was not added, and the redispersibility was evaluated. The results are shown in Table 1.

(Comparative Examples 2 and 3)
Carboxymethyl cellulose (CMC, trade name "Selogen 7A", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) or hydroxyethyl cellulose (HEC, trade name "AL-15F", manufactured by Sumitomo Seika Chemical Co., Ltd.) was added in place of trimethylglycine. A dispersion was prepared by the same operation as in Example 4, and the redispersibility was evaluated. The results are summarized in Table 1.

As shown in Table 1, the redispersions of the cellulose nanofibers of Examples 1 to 7 prepared by adding the betaine compound showed a viscosity close to that of the dispersion before drying, and had high redispersability. It is considered that this is because the betaine compound suppressed the aggregation of the cellulose nanofibers during drying.

On the other hand, in Comparative Example 1 to which the betaine compound was not added, the viscosity after redispersion was lower than that before drying. It is considered that this is because the cellulose nanofibers are strongly aggregated due to the hydrogen bonds between the hydroxyl groups on the surface of the cellulose nanofibers. Further, Comparative Examples 2 and 3 were inferior to Example 4 to which the same amount of betaine compound was added, although the redispersibility was improved as compared with Comparative Example 1 by adding the water-soluble cellulose derivative.

(Example 8)
Pure water was added to the aqueous dispersion of anion-modified cellulose nanofibers in the same manner as in Example 1 and diluted to a solid content concentration of 0.4% by mass. To this diluted solution, 100 parts by mass of trimethylglycine was added to 100 parts by mass of cellulose nanofibers, and the mixture was stirred and dissolved. Then, the dispersion was freeze-dried at −22 ° C. to obtain a dried cellulose nanofiber composition.

(Example 9)
A dried cellulose nanofiber composition was obtained in the same manner as in Example 8 except that 500 parts by mass of trimethylglycine was added to 100 parts by mass of the cellulose nanofibers.

(Comparative Example 4)
A dried cellulose nanofiber composition was obtained in the same manner as in Example 8 except that trimethylglycine was not added.

0.01 g of the dried products of Examples 8 and 9 and Comparative Example 4 were dispersed in 10 ml of water, 20 μl was taken from the dispersion, and diluted 50 times to prepare a sample for atomic force microscope (AFM) observation. 20 μl of the observation sample was dropped on the mica substrate and air-dried, and the sample was observed by AFM. The AFM images of Example 8, Example 9, and Comparative Example 4 are shown in FIGS. 1 to 3, respectively.

As a result of AFM observation, in Example 8 (FIG. 1), although some entanglement of the modified cellulose nanofibers was observed, many of them were redispersed to a single nanosize, and there were few aggregates. Moreover, in Example 9 (FIG. 2), many of them were redispersed to a single nano size. In addition, it was observed that the betaine compound coated the modified cellulose nanofibers. Almost no aggregates of modified cellulose nanofibers were observed.

On the other hand, in Comparative Example 4 (Fig. 3) to which the betaine compound was not added, there was a portion redispersed to a single nanosize, but large entanglements and agglomerates between the modified cellulose nanofibers were observed.

(Example 10)
Pure water was added to the aqueous dispersion of anion-modified cellulose nanofibers in the same manner as in Example 1 and diluted to a solid content concentration of 0.4% by mass. To this diluted solution, 100 parts by mass of trimethylglycine was added to 100 parts by mass of cellulose nanofibers, and the mixture was stirred and dissolved. Then, the dispersion was placed in a constant temperature bath at 105 ° C. and allowed to stand until the mass became constant to obtain a dry cellulose nanofiber composition.

(Comparative Example 5)
A dried cellulose nanofiber composition was obtained in the same manner as in Example 10 except that trimethylglycine was not added.

0.01 g of the dried product of Example 10 and Comparative Example 5 was dispersed in 10 ml of water, 20 μl was taken from the dispersion, and diluted 50 times to prepare a sample for atomic force microscope (AFM) observation. 20 μl of the observation sample was dropped on the mica substrate and air-dried, and the sample was observed by AFM. The AFM images of Example 10 and Comparative Example 5 are shown in FIGS. 4 and 5, respectively.

As a result of AFM observation, in Example 10 (FIG. 4), although some entanglement of the modified cellulose nanofibers was observed, many of them were redistributed to a single nanosize, and the modified cellulose nanofibers had a long fiber length. Was observed.

On the other hand, in Comparative Example 5 (FIG. 5) to which the betaine compound was not added, there was a portion redispersed to a single nanosize, but thick aggregates and large entanglements of the modified cellulose nanofibers accounted for the majority.

Claims

A cellulose nanofiber composition containing a modified cellulose nanofiber having an ionic functional group and a betaine compound having a molecular weight of 600 or less.
The cellulose nanofiber composition according to claim 1, wherein the content of the betaine compound with respect to 100 parts by mass of the modified cellulose nanofiber is 5 to 1000 parts by mass.
The cellulose nanofiber composition according to claim 1 or 2, which is a dried product.
The cellulose nanofiber composition according to claim 1 or 2, which comprises a dispersion medium and in which the modified cellulose nanofibers are dispersed in the dispersion medium.
A method for producing a dry cellulose nanofiber composition, which comprises a step of drying a dispersion of modified cellulose nanofibers having an ionic functional group and containing a betaine compound having a molecular weight of 600 or less.
The method for producing a cellulose nanofiber composition according to claim 5, wherein the drying step is a heat-drying or freeze-drying step.